1. Field of the Invention
The present invention relates generally to a digital multimedia broadcasting (DMB), and in particular, to a data receiving device and method for shortening a channel switching time in a DMB system.
2. Description of the Related Art
In general, digital broadcasting refers to a broadcasting service for providing a high definition, high fidelity, and high quality service to a user.
A satellite digital broadcasting, mainly aimed at a mobile service can allow a user to view a multi-channel broadcasting anytime and anywhere, using a mobile receiving device, e.g., a mobile phone, a personal digital assistant, and a vehicle receiving device.
A terrestrial digital broadcasting system, whose parent is digital audio broadcasting (DAB), is based on a concept of providing a mobile receiving multimedia broadcasting using a 12-numbered channel of a very high frequency (VHF) that is currently in idle. The terrestrial digital broadcasting transmits a television broadcast, a radio broadcast, and a data broadcast. Existing terrestrial service providers manage one analog channel, but digital broadcasting service providers are based on a concept of managing a plurality of digital channels. These services are called an ensemble.
In the terrestrial digital broadcasting, three ensembles can be transmitted over a VHF channel. It is possible to provide the service over about one video channel, two audio channels, and one data channel per ensemble.
Owing to the developments of a digital broadcasting technology and a mobile communication technology, attention is increasingly paid to a digital broadcasting service for allowing the user to view the digital broadcast even when the user is moving. In particular, attention is increasingly paid to a digital multimedia broadcasting (DMB) service using a mobile terminal.
Thus, the users can view a desired broadcasting anytime and anywhere, without being held to a fixed time, by storing the digital broadcast received using a DMB service in the mobile terminal.
However, the digital broadcast received using the DMB service has a drawback in that a channel switching time is remarkably longer compared with that of a general, analog, television broadcast.
FIG. 1 illustrates an example of a delay generated at the time of a channel switch in a satellite DMB service.
As shown in FIG. 1, five delays occur in a process of responding to a channel switching signal of the satellite DMB service from the user, switching a channel of the satellite DMB service, and outputting A/V data.
The five delays occurring at the time of the channel switch will be described in more detail. A deinterleaver buffering delay 101 refers to a time taken to empty and again fill a deinterleaver at the time of changing a code division multiplexing (CDM) channel in the satellite DMB service.
In general, the satellite DMB service is of a type in which broadcast programs are dispersed and transmitted to an N number of CDM channels. Each CDM channel is classified into a pilot channel, a program specific information (PSI) and system information (SI) channel, and a video and voice channel.
In the satellite DMB service, broadcast traffic is interleaved and transmitted on a very large unit. In other words, since traffic information encoded by an encoder is interleaved and transmitted on the very large unit, a receiving device cannot perform decoding until it receives all the interleaved traffic information. Further the receiver cannot perform the decoding until it completes the deinterleaving. Accordingly, in the satellite DMB service, the receiving device cannot decode broadcast service traffic until it receives data of a known amount interleaved before decoding the received traffic. Therefore, a significant delay is caused in providing the service.
A conditional access system (CAS) delay 102 refers to a time for waiting until necessary information is transmitted to provide a CAS service for checking whether or not a subscriber is authenticated.
p The deinterleaver buffering delay 101 and the CAS delay 102 are called a network delay 106.
A video key frame extraction delay 103 refers to a time for waiting until a key frame, for example, an instantaneous decoder refresh (IDR) frame in H.264, is reached to decode a compressed image data in a normal fashion.
Generally, in a compression method like H.264, there are three frame types;, intra-coded (I), predictive coded (P), and bi-directionally predictive coded (B) frames depending on a method for deciding a reference frame. The I frame refers to a frame for simply spatially converting only the frame without a motion compensation. The P frame refers to a frame for performing the motion compensation in a forward direction or in a reverse direction with reference to the I frame or the P frame and then, spatially converting a residual. The B frame refers to a frame for performing the motion compensation like the P frame but performing the motion compensation by the I and P frames, that is, in both directions on a time axis.
A frame encoding method, in which an inputted image can be restored to the original state independently of adjacent other images like the I frame, is called an original video encoding. An encoding method for estimating a current image by a previous image with reference to front/rear neighboring I frame or adjacent P frame is called a residual video encoding.
The key frame is a complete picture used for video compression. Referring to video group of pictures (GOP) structure, a frame is selected one by one at a constant interval from a temporal video flow, and is designated as the key frame. The key frame is an independently restorable video, and enables an arbitrary video access.
In MPEG, H.261, and H.264, the key frame is inserted at a constant interval into a GOP, and designates the I frame enabling independent video reproduction. However, without limitation to this, the frame independently restored to the original state without reference to other frames can be all defined as the key frame irrespective of a moving picture compression method.
An A/V reproducing buffer delay 104 refers to a delay time for buffering a predetermined amount of A/V data to stably reproduce the A/V data.
An A/V decoding delay 105 refers to a delay time for decoding and outputting the A/V data.
The video key frame extraction delay 103, the A/V reproducing buffer delay 104, and the A/V decoding delay 105 are called a codec delay 107.
In general, the deinterleaver buffering delay 101 generates a delay time of 2 seconds to 3 seconds. The video key frame extraction delay 103 generates a delay time of 0.5 seconds to 2 seconds. The A/V reproducing buffer delay 104 generates a delay time of about 1 second.
In the conventional DMB service, a technology for shortening a channel switching delay time mostly uses a method for providing several CDM channels and previously filling the deinterleaver of the adjacent channel.
This can minimize the deinterleaver buffering delay 101 at the time of the channel switching to the adjacent channel. Comparing with the deinterleaver buffering delay 101, the codec delay 107 is a long delay time. Therefore, a fast channel switching is impossible until the time for the codec delay 107 is shortened.
However, the above method can shorten a time for the deinterleaver buffering delay 101 of the network delay 106, but cannot shorten a time for the video key frame extraction delay 103 and the A/V reproducing buffer delay 104 of the codec delay 107.